This invention relates generally to an elongated shaft, and more particularly concerns forming an elongated plastic shaft with a stepped functional surface. Also described is a gear with a stepped hub that runs on the stepped functional surface of the shaft.
Currently, when molding long plastic shafts a 2-5 degree draft along the length of the shaft is necessary in order for the shaft to be successfully removed from the mold. This means that any gear to be used on the shaft must have its hub formed to allow for the degree of draft of the shaft. Even with the gear designed for the draft of the shaft, there is a sloppy fit between the shaft and gear which results in rattling noise and uneven wear.
FIG. 1 shows an elongated shaft 10 with a base 12, and first section 14 which has a second section 16 connected thereto. The shaft shown is hollow as indicated by bore 18 and formed by an injection molding or extrusion process, however, the shaft may also be solid. Typical plastic materials include thermoplastic and thermosetting resins. Typical thermoplastic resins include polyethylene, polystyrene, polypropylene, polyurethane, polyvinylchloride, nylons, polycarbonate ABS, as well as fluorcarbons. Typical thermosetting resins include acrylics, phenolics and polyesters. In conventional molding processes in order for the molded shaft to be separated from the mold section 14 must have a draft of 2-5 degrees as shown. Gear 20 runs on first section 14 and the gear's hub 22 is designed with a draft corresponding to the draft of the shaft. These angled surfaces of the shaft and the gear do not allow for a tight fit since precise forming of angled surfaces is difficult. The angled gear and shaft configuration has axial forces acting the surfaces where the gear hub and shaft surface come into contact.
A patent teaching a molded shaft with different molded feature is U.S. Pat. No. 5,439,416. The shaft assembly is comprised of an elongated member having a hollow, tubular shell with a hardened moldable material filling the shell and a functional feature molded on the outer surface of the shaft. A gear may be the functional feature integrally molded with the shaft. The teachings of this patent are hereby incorporated by reference.